The following background describes one context in which embodiments of the present invention may be practiced and should not be viewed as limiting the scope of the present invention as set forth in the appended claims.
Tachycardia is the condition of an accelerated pulse rate. Natural tachycardia occurs in physical exercise and emotional stress because of the sympathetic nervous tone and increase of the circulatory catecholamine concentration. The most important quality of natural tachycardia is the resultant increase in cardiac output. Pathologic tachycardia results in impaired hemodynamics, i.e. decreased cardiac output. The electrophysiology discriminates two major classes of tachycardia: supra-ventricular and ventricular, as well as two major classes of etiology: ectopic focuses and reentry phenomena. The therapy of tachycardia is in principle either the suppression of ectopic focuses or interruption of the reentry pathway. The first approach is always the pharmacotherapy. Despite of the recent advances in electropharmacology, every antiarrhythmic drug is not effective in every patient. Drugs also provoke side effects, which can be hazardous to the patient. Therefore more invasive modes of therapy must be considered like surgical treatment and permanent implantation of an electrotherapy device. One therapy of choice is cardiac ablation, which is a semi-invasive interventional method.
Transvenous catheter ablation of cardiac conduction tissue is a low risk alternative to surgical ablation to treat refractory supra-ventricular tachyarrhythmias. Some positive results have also been achieved in the treatment of ventricular tachycardia. The principal energy source for catheter ablations is a DC energy pulse from a standard defibrillator. In order to minimize the energy for the purpose of safety, numerous modifications in design of the energy source as well as of the catheter have been realized. In order to achieve the controllability of the lesion size as well as to avoid hazardous shock wave, the radiofrequency energy source has been introduced. For the same reason, laser ablation fiber optic catheters have been developed. The application of microwave energy is an alternative method, as well as ablation by means of chemical agents.
One of the challenges in to a clinician in performing an ablation procedure is the exact positioning of the ablation electrode within the heart. The positioning is normally observed under of radiographic imaging. Limitations of X-ray methods, however, include poor imaging of soft tissues, i.e. papillary muscle, interventricular septa, and so forth. As an alternative, ultrasonic imaging is well suited for imaging of soft tissues. Ultrasonically marked catheters and cardiac pacing leads have been described in U.S. Pat. No. 4,697,595 (Breyer, et al.) and in U.S. Pat. No. 4,706,681 (Breyer, et al.) respectively. Such systems enable the echocardiography guidance of the procedure for deploying a lead as well as the exact localization of the lead tip. If an ultrasonic transducer marks an ablation electrode, the exact position of the ablation electrode can be identified. A system having an ultrasonically marked cardiac ablation electrode wherein the ultrasonic sensitivity characteristics may be either in the same direction as the ablation field or in some other direction is disclosed in U.S. Pat. No. 5,840,030 (Ferek-Petric et al.), which is incorporated herein by reference in its entirety. The system allows radial orientation of the ultrasonically marked catheter as wells as directional field ablation.
Electrophysiologists usually monitor the intracardiac potentials to confirm the proper position as well as the proper contact of the electrode with the endocardium. However, the intracardiac potential is discontinuous being characterized with intrinsic deflection, which is repetitive at the frequency of the heartbeats. Distinct ST elevation caused by the injury current confirms the pressure of the electrode to the cardiac muscle. However, dislodgement may also occur anywhere within the cardiac cycle while there is no intracardiac signal. Ultrasonic imaging of the cardiac tissues and an ultrasonically marked ablation catheter allow the distance between the tissue and the ablation electrode to be measured as disclosed in the above-incorporated Ferek-Petric patent.
Although three-dimensional ultrasonic imaging is available, the currently high cost of three-dimensional systems generally prohibits widespread use. Therefore, even with the assistance of ultrasonically marked catheter, the catheter position is generally displayed in two dimensions. The clinician is required to envision the catheter position in three-dimensions, which makes exact positioning of the catheter a challenge. Three dimensional mapping systems have been proposed including the mapping system and method disclosed in U.S. Pat. No. 5,697,377 (Wittkampf et al.), which is incorporated herein by reference in its entirety. In this system, a catheter is provided with at least one measuring electrode. A voltage is measured between the measuring electrode and a reference electrode, which voltage signal has components corresponding to three orthogonal current signals applied to the patient substantially in the area to be mapped, such as the heart. The three-dimensional location of the catheter within the patient's body may be determined from the measured voltage signal components. This three-dimensional location may be represented relative to reference points on a graphic user interface. Without an anatomical image, however, the clinician must still envision the three-dimensional location of the catheter with respect to the patient's anatomy.